1. Field of the Invention
This invention relates generally to oilfield wellbore systems for performing wellbore operations and more particularly to subsea downhole operations at an offshore location in which drilling fluid is continuously circulated through the wellbore and which utilizes a fluid return line that extends from subsea wellhead equipment to the surface for returning the wellbore fluid from the wellhead to the surface. Maintenance of the fluid pressure in the wellbore during drilling operations at predetermined pressures is key to enhancing the drilling operations.
2. Background of the Art
Oilfield wellbores are drilled by rotating a drill bit conveyed into the wellbore by a drill string. The drill string includes a drilling assembly (also referred to as the xe2x80x9cbottom hole assemblyxe2x80x9d or xe2x80x9cBHAxe2x80x9d) that carries the drill bit. The BHA is conveyed into the wellbore by tubing. Continuous tubing such as coiled tubing or jointed tubing is utilized to convey the drilling assembly into the wellbore. The drilling assembly usually includes a drilling motor or a xe2x80x9cmud motorxe2x80x9d that rotates the drill bit. The drilling assembly also includes a variety of sensors for taking measurements of a variety of drilling, formation and BHA parameters. A suitable drilling fluid (commonly referred to as the xe2x80x9cmudxe2x80x9d) is supplied or pumped under pressure from the surface down the tubing. The drilling fluid drives the mud motor and discharges at the bottom of the drill bit. The drilling fluid returns uphole via the annulus between the drill string and the wellbore inside and carries pieces of formation (commonly referred to as the xe2x80x9ccuttingsxe2x80x9d) cut or produced by the drill bit in drilling the wellbore.
For drilling wellbores under water (referred to in the industry as xe2x80x9coffshorexe2x80x9d or xe2x80x9csubseaxe2x80x9d drilling) tubing is provided at the surface work station (located on a vessel or platform). One or more tubing injectors or rigs are used to move the tubing into and out of the wellbore. Injectors may be placed at the sea surface and/or on the wellhead equipment at the sea bottom. In riser-type drilling, a riser, which is formed by joining sections of casing or pipe, is deployed between the drilling vessel and the wellhead equipment and is utilized to guide the tubing to the wellhead. The riser also serves as a conduit for fluid returning from the wellhead to the sea surface. Alternatively, a return line, separate and spaced apart from the tubing, may be used to return the drilling fluid from the wellbore to the surface.
During drilling, the operators attempt to carefully control the fluid density at the surface so as to ensure an overburdened condition in the wellbore. In other words, the operator maintains the hydrostatic pressure of the drilling fluid in the wellbore above the formation or pore pressure to avoid well blow-out. The density of the drilling fluid and the fluid flow rate control largely determine the effectiveness of the drilling fluid to carry the cuttings to the surface. For such purpose, one important downhole parameter controlled is the equivalent circulating density (xe2x80x9cECDxe2x80x9d) of the fluid at the wellbore bottom. The ECD at a given depth in the wellbore is a function of the density of the drilling fluid being supplied and the density of the returning fluid which includes the cuttings at that depth.
When drilling at offshore locations where the water depth is a significant fraction of the total depth of the wellbore, the absence of a formation overburden causes a reduction in the difference between pore fluid pressure in the formation and the pressure inside the wellbore due to the drilling mud. In addition, the drilling mud must have a density greater than that of seawater so then if the wellhead is open to seawater, the well will not flow. The combination of these two factors can prevent drilling to certain target depths when the fill column of mud is applied to the annulus. The situation is worsened when liquid circulation losses are included, thereby increasing the solids concentration and creating an ECD of the return fluid even greater than the static mud weight.
In order to be able to drill a well of this type to a total wellbore depth at a subsea location, the bottom hole ECD must be reduced. One approach to do so is to use a mud filled riser to form a subsea fluid circulation system utilizing the tubing, BHA, the annulus between the tubing and the wellbore and the mud filled riser, and then inject gas (or some other low density liquid) in the primary drilling fluid (typically in the annulus adjacent the BHA) to reduce the density of fluid downstream (i.e., in the remainder of the fluid circulation system). This so-called xe2x80x9cdual densityxe2x80x9d approach is often referred to as drilling with compressible fluids.
Another method for changing the density gradient in a deepwater return fluid path has been proposed, but not used in practical application. This approach proposes to use a tank, such as an elastic bag, at the sea floor for receiving return fluid from the wellbore annulus and holding it at the hydrostatic pressure of the water at the sea floor. Independent of the flow in the annulus, a separate return line connected to the sea floor storage tank and a subsea lifting pump delivers the return fluid to the surface. Although this technique (which is referred to as xe2x80x9cdual gradientxe2x80x9d drilling) would use a single fluid, it would also require a discontinuity in the hydraulic gradient line between the sea floor storage tank and the subsea lifting pump. This requires close monitoring and control of the pressure at the subsea storage tank, subsea hydrostatic water pressure, subsea lifting pump operation and the surface pump delivering drilling fluids under pressure into the tubing for flow downhole. The level of complexity of the required subsea instrumentation and controls as well as the difficulty of deployment of the system has delayed (if not altogether prevented) the practical application of the xe2x80x9cdual gradientxe2x80x9d system.
The present invention provides wellbore systems for performing subsea downhole wellbore operations, such as subsea drilling as described more fully hereinafter, as well as other wellbore operations, such as wellbore reentry, intervention and recompletion. Such drilling system includes tubing at the sea level. A rig at the sea level moves the tubing from the reel into and out of the wellbore. A bottom hole assembly, carrying the drill bit, is attached to the bottom end of the tubing. A wellhead assembly at the sea bottom receives the bottom hole assembly and the tubing. A drilling fluid system continuously supplies drilling fluid into the tubing, which discharges at the drill bit and returns to the wellhead equipment carrying the drill cuttings. A pump at the surface is used to pump the drilling fluid downhole. A fluid return line extending from the wellhead equipment to the surface work station transports the returning fluid to the surface.
In the preferred embodiment of the invention, an adjustable pump is provided coupled to the annulus of the well. The lift provided by the adjustable pump effectively lowers the bottom hole pressure. In an alternative embodiment of the present invention, a flowable material, whose fluid density is less than the density of the returning fluid, is injected into a return line separate and spaced from the tubing at one or more suitable locations in the return line or wellhead. The rate of injection of such lighter material can be controlled to provide additional regulation of the pressure the return line and to maintain the pressure in the wellbore at predetermined values throughout the tripping and drilling operations.
Some embodiments of the drilling system of this invention are free of subsea risers that usually extend from the wellhead equipment to the surface and carry the returning drilling fluid to the surface. Fluid flow control devices may also be provided in the return line and in the tubing. Sensors make measurements of a variety of parameters related to conditions of the return fluid in the wellbore. These measurements are used by a control system, preferably at the surface, to control the surface and adjustable pumps, the injection of low density fluid at a controlled flow rate and flow restriction devices included in the drilling system. In other embodiments of the invention, subsea risers are used as guide tubes for the tubing and a surge tank or stand pipe in communication with the return fluid in the flow of the fluid to the surface.
These features (in some instances acting individually and other instances acting in combination thereof) regulate the fluid pressure in the borehole at predetermined values during subsea downhole operations in the wellbore by operating the adjustable pump system to overcome at least a portion of the hydrostatic pressure and friction loss pressure of the return fluid. Thus, these features enable the downhole pressure to be varied through a significantly wider range of pressures than previously possible, to be adjusted far faster and more responsively than previously possible and to be adjusted for a wide range of applications (i.e., with or without risers and with coiled or jointed tubing). In addition, these features enable the bottom hole pressure to be regulated throughout the entire range of downhole subsea operations, including drilling, tripping, reentry, recompletion, logging and other intervention operations, which has not been possible earlier. Moreover, the subsea equipment necessary to effect these operational benefits can be readily deployed and operationally controlled from the surface. These advantages thus result in faster and more effective subsea downhole operations and more production from the reservoir, such as setting casing in the wellbore.
Examples of the more important features of the invention have been summarized (albeit rather broadly) in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.